Before discussing the individual hormones of the adenohypophysis, it is important to understand the structural •Fig. 41.2 A,Grossstructureofthepituitarygland.Thepituitaryglandisbelowthehypothalamusandisconnectedtoitbythepituitarystalk.Theglandsitswithinthesellaturcica,afossawithinthesphenoidbone,andiscoveredbyaduralreflection,thediaphragmasellae.Theparsdistalismakesupmostoftheanteriorpituitary.B,Theparsdistalisisderivedfromepithelialtissuethatiscomposedofacidophils (A)(somatotropesandlactotropes)andbasophils(B)(thyrotropes,gonadotropes,andcorticotropes).C,Theposteriorpituitaryisderivedfromneuraltissueandhasahistologicalappearanceofnonmyelinatednerves.Cp,chromophobes;H,Herringbodies.(A,ModifiedfromStevensA.In:LoweJS[ed].Human Histology. 3rded.Philadelphia:Elsevier;2005.BandC,FromYoungBetal[eds].Wheater’s Functional Histology. 5thed.Philadelphia:ChurchillLivingstone;2006.) •Fig. 41.3 Magnocellularneuronsofthehypothalamus(paraven-tricularandsupraopticnuclei)projecttheiraxonsdowntheinfundibularprocessandterminateintheparsnervosa(posteriorlobe),wheretheyreleasetheirhormones(eitherADHoroxytocin)intoacapillarybed.(ModifiedfromLarsenPRetal[eds].Williams Textbook of Endocrinol-ogy. 10thed.Philadelphia:Saunders;2003.)Capilary bed Anterior lobe of pituitary Optic chiasm Hypothalamus Posterior lobe of pituitary Magnocellular neurosecretory cells
Becauseposteriorpituitaryhormonesaresynthesizedinthehypothalamusratherthanthepituitary,hypophysectomy (pituitaryremoval)doesnotnecessarilypermanentlydisruptsynthesisandsecretionofthesehormones.Immediatelyafterhypophysectomy,secretionofthehormonesdecreases.However,overaperiodofweekstheseveredproximalendofthetractwillshowhistologicalmodificationandpituicyteswillformaroundtheneuronterminals.Secretoryvacuolesareseen,andsecretionofhormoneresumesfromthisproximalend.Secretionofhormonecanevenpotentiallyreturntonormallevels.Incontrast,alesionhigheruponthepituitarystalkcanleadtolossofneuronalcellbodiesinthePVNandSON.
and functional organization of the adenohypophysis in the context of the endocrine axes (
Fig. 41.6 41.1 ). Each endocrine axis is composed of three levels of endocrine cells: (1) hypothalamic neurons, (2) anterior pituitary cells, and (3) peripheral endocrine glands. Hypothalamic neurons release specific hypothalamic releasing hormones (designated XRH in this generic scheme) that stimulate secretion of specific pituitary tropic hormones (XTH). In some cases, production of a pituitary tropic hormone is secondarily regulated by a release-inhibiting hormone (XIH). Pituitary tropic hormones then act on specific peripheral target endocrine glands and stimulate them to release peripheral hormones (X). The peripheral hormone X has two general functions: it regulates several aspects of human physiology, and it negatively feeds back on the pituitary gland and hypothalamus to inhibit production and secretion of tropic hormones and releasing hormones, respectively (see
Fig. 41.6